Pump end of stroke sensor

ABSTRACT

A prime mover includes a shaft assembly that is supported for reciprocal movement between first and second ends of a stroke, and that includes first and second portions having a first material, and a third portion in between the first and second portions, having a second material, different from the first material. A single inductive proximity sensor makes a first indication when proximate the first material, and makes a second indication, when proximate the second material. A portion of the sensor is proximate the first portion when the shaft is at the first end of the stroke, the same portion of the same sensor is proximate the second portion when the shaft is at the second end of the stroke, and the same portion of the same sensor is proximate the third portion the entire time the shaft is in between the first and second ends of the stroke.

RELATED APPLICATION

This application claims priority to Provisional Patent Application No. 61/148,840, filed Jan. 30, 2009, the content of which is herein incorporated by reference.

BACKGROUND

The present invention relates to reciprocating pumps, motors, or other machinery having a part that moves between first and second end positions, and a sensor for determining when the first and second ends of the stroke have been achieved.

SUMMARY

In one embodiment, the invention provides a prime mover including a shaft assembly supported for reciprocal movement between first and second ends of a stroke, the shaft assembly includes first and second portions having a first material, and a third portion in between the first and second portions, having a second material different from the first material. A single inductive proximity sensor makes a first indication in response to being proximate the first material, and makes a second indication, different from the first indication, in response to being proximate the second material. A portion of the single inductive proximity sensor is proximate the first portion of the shaft assembly when the shaft is at the first end of the stroke, the same portion of the same single inductive proximity sensor is proximate the second portion of the shaft assembly when the shaft is at the second end of the stroke, and the same portion of the same single inductive proximity sensor is proximate the third portion of the shaft assembly the entire time the shaft is in between the first and second ends of the stroke.

In another embodiment, the invention provides a method of operating a machine. The method includes providing a shaft assembly having first and second portions each having a first material and a third portion having second material different from the first material, providing a drive assembly operable to drive reciprocation of the shaft assembly between first and second opposite ends of a stroke and providing a single inductive proximity sensor. The method further includes positioning the first portion of the shaft assembly proximate the single inductive proximity sensor when the shaft assembly is at the first end of the stroke, positioning the second portion of the shaft assembly proximate the single inductive proximity sensor when the shaft assembly is at the second end of the stroke, and positioning the third portion of the shaft assembly proximate the single inductive proximity sensor the entire time the shaft assembly is in between the first and second ends of the stroke. The method further includes making a first indication with the single inductive proximity sensor in response to the single inductive proximity sensor being proximate the first and second portions of the shaft assembly, making a second indication, different from the first indication, with the single inductive proximity sensor in response to the single inductive proximity sensor being proximate the third portion of the shaft assembly, and actuating the drive assembly to reverse a direction of movement of the shaft assembly in response to the single inductive proximity sensor making the first indication.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a first perspective view of a double diaphragm pump according to first and second embodiments of the present invention.

FIG. 2 is a second perspective view of the double diaphragm pump of FIG. 1.

FIG. 3 is a cross-sectional view of the first embodiment of the pump along line 3-3 of FIG. 1.

FIG. 4 is a cross-sectional view of the first embodiment of the pump along line 4-4 of FIG. 1 showing a shaft in a neutral position.

FIG. 5 is a cross-sectional view of the first embodiment of the pump along line 4-4 of FIG. 1 showing a shaft in a first end position.

FIG. 6 is a cross-sectional view of the first end of the pump along line 4-4 of FIG. 1 showing a shaft in a second end position.

FIG. 7 is a schematic view of a control system for the double diaphragm pump of FIG. 1.

FIG. 8 is a cross-sectional view of the second embodiment of the pump along line 3-3 of FIG. 1.

FIG. 9 is a cross-sectional view of the second embodiment of the pump along line 4-4 of FIG. 1 showing a shaft in a neutral position.

FIG. 10 is a cross-sectional view of the second embodiment of the pump along line 4-4 of FIG. 1 showing a shaft in a first end position.

FIG. 11 is a cross-sectional view of the second embodiment of the pump along line 4-4 of FIG. 1 showing a shaft in a second end position.

FIG. 12 is a schematic view of a control system for the double diaphragm pump of FIG. 1.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings.

FIGS. 1-3 illustrate a prime mover, such as a double diaphragm pump 10 having a housing defining two working chambers 15 a, 15 b. In other embodiments, the prime mover may be another type of double-diaphragm pump, a piston pump, a motor or any other machinery having a reciprocating or moving part to be monitored. Each working chamber 15 a, 15 b is divided with a flexible diaphragm (first diaphragm 20 a, second diaphragm 20 b) into a respective pumping chamber 25 a, 25 b and a respective motive fluid chamber 30 a, 30 b. The diaphragms 20 a, 20 b include respective diaphragm washers 22 a, 22 b. The diaphragms 20 a, 20 b are interconnected through a shaft 35 for synchronized reciprocating movement, such that when one diaphragm 20 a, 20 b is moved to increase the volume of the associated pump chamber 25 a, 25 b, the other diaphragm 20 a, 20 b is simultaneously moved to decrease the volume of the associated pump chamber 25 a, 25 b. The shaft 35 illustrated in FIG. 3 is a reciprocating rod. The pump 10 includes at least one inlet 40 a, 40 b (FIG. 4) for the supply of a motive fluid 42 (e.g., compressed air or another pressurized gas) and a valve 45 for alternatingly supplying the motive fluid to the motive fluid chambers 30 a, 30 b to drive reciprocation of the first and second diaphragms 20 a, 20 b and the shaft 35. Simultaneously with supplying the motive fluid 42 to one of the motive fluid chambers 30 a, 30 b, the valve 45 places an exhaust assembly 50 in communication with the other motive fluid chamber 30 a, 30 b to permit motive fluid 42 to be expelled therefrom. The inlet 40 a is in fluid communication with the pump chamber 30 a when the inlet 40 b is in fluid communication with the exhaust assembly 50 when the shaft 35 is in a first position. Likewise, the inlet 40 b is in fluid communication with the pump chamber 30 b when the inlet 40 a is in fluid communication with the exhaust assembly 50 when the shaft 35 is in the second position.

In operation, as the diaphragms 20 a, 20 b and shaft 35 reciprocate, the pump chambers 25 a, 25 b alternatingly expand and contract to create respective low and high pressure within the respective chambers 25 a, 25 b. The pump chambers 25 a, 25 b communicate with an inlet manifold 55 that is connected to a reservoir containing a fluid to be pumped, and also communicate with an outlet manifold 60 that is connected to a receptacle for the fluid being pumped. Check valve assemblies 65 ensure that the fluid being pumped moves only from the inlet manifold 55 toward the outlet manifold 60. When one of the pump chambers 25 a, 25 b expands, the resulting negative pressure causes the upper check valve assembly 65 associated with the pump chamber 25 a, 25 b to close, causes the lower check valve assembly 65 associated with the pump chamber 25 a, 25 b to open, and draws fluid from the inlet manifold 55 into the pump chamber 25 a, 25 b. Simultaneously, the other pump chamber 25 a, 25 b contracts, which creates positive pressure that causes the lower check valve assembly 65 associated with the pump chamber 25 a, 25 b to close, causes the upper check valve assembly 65 associated with the pump chamber 25 a, 25 b to open, and forces fluid into the outlet manifold 60.

The shaft 35 has a fixed length, such that the position of the shaft 35 in the pump 10 is indicative of the position of the diaphragms 20 a, 20 b. The shaft 35 and diaphragms 20 a, 20 b move back and forth a fixed distance that defines a stroke. The fixed distance is determined by the geometry of the pump 10, the shaft 35, the diaphragms 20 a, 20 b, and the diaphragm washers 22 a, 22 b. The stroke is defined as the travel path of the shaft 35 between a first end position and a second end position (see FIGS. 5 and 6) that define respective first and second ends of the shaft's stroke. Movement of the shaft 35 from one end of the stroke to the other end of the stroke and back defines a cycle of operation of the shaft 35 (i.e., a cycle consists of two consecutive strokes). The first and second end positions occur when the diaphragms 20 a, 20 b reach the end of movement or the diaphragm washers 22 a, 22 b bottom out, thereby causing the shaft 35 to stop and travel in a reverse direction. In the first end position, illustrated in FIG. 5, the first diaphragm 20 a minimizes the volume of the motive fluid chamber 30 a, and the second diaphragm 20 b minimizes the volume of the pump chamber 25 b. In the second end position, illustrated in FIG. 6, the first diaphragm 20 a minimizes the volume of the pump chamber 25 a, and the second diaphragm 20 b minimizes the volume of the motive fluid chamber 30 b.

Referring now to FIGS. 1-6, and 8-11, the pump 10 includes a sensor 110, which in the illustrated embodiments is an inductive proximity sensor. In other embodiments other similar proximity sensors can be utilized. The sensor 110 senses the presence or absence of metallic material proximate the sensor 110. One example of a suitable proximity sensor is part number AM1-AN-2H available from Automation Direct. Information on this and other similar proximity sensors is shown in the Automation Direct Catalog, pages 17-19 through 17-21 available at www.automationdirect.com/proximity. Suitable metallic materials that can be utilized with a proximity sensor are iron (FE37), stainless steel, brass, bronze, aluminum and copper. When the metallic material is proximate the sensor 110, the sensor 110 generates a proximity signal. When the metallic material is not proximate the sensor 110, the sensor 110 does not generate a signal. The term “proximate,” “near,” “adjacent,” and similar terms as used with respect to the sensor 110 and another part of the pump 10, mean that the sensor 110 is very close to, but not in physical contact with, another the other part of the pump 10.

With reference to the first embodiment in FIGS. 3-6, the shaft 35 includes a metallic ring 115 that is either integrally formed with the shaft 35 or that is affixed to the shaft 35. The metallic ring 115 has a greater diameter than the shaft 35. When the shaft 35 is in between the ends of its stroke (as illustrated in FIG. 4), the sensor 110 is proximate the metallic ring 115 and generates the proximity signal in response to the presence of the metallic ring 115. A portion of the sensor 110 remains proximate the metallic ring 115 as the shaft 35 moves, until the shaft 35 actually reaches the first and second ends of its stroke. Thus, the metallic ring 115 is proximate the sensor 110 the entire time that the shaft 35 is in between the ends of its stroke.

When the shaft 35 reaches the first and second ends of its stroke (as illustrated in FIGS. 5 and 6), the sensor 110 is no longer proximate the metallic ring 115 because no portion of the ring is near the end of the sensor 110. The sensor 110 is spaced from the metallic ring 115, and thus does not sense the presence of the metallic ring 115. Additionally, the shaft 35 is not proximate the sensor 110 when the shaft 35 is at the first and second ends of its stroke, because the shaft 35 has a smaller diameter than the metallic ring 115 and the resulting air gap prevents the sensor 110 from sensing the shaft 35, even if the shaft is made of metallic material. Thus, in this embodiment, the sensor 110 generates the proximity signal the entire time that the shaft 35 is in between the first and second ends of the stroke, and the same sensor 110 does not generate the proximity signal when the shaft 35 reaches the ends of its stroke.

In this regard, the shaft assembly of the first embodiment includes the shaft 35, the ring 115 and the air gaps on each side of the ring 115 between the proximity sensor 110 and the shaft 35. The proximity sensor 110 is therefore proximate first and second portions of the shaft assembly (i.e., the air gap and portions of the shaft 35 on either side of the ring 115) when the shaft assembly is at the first and second ends of the stroke, and is proximate a third portion of the shaft assembly (i.e., the ring 115 and the portion of the shaft 35 covered by the ring 115) the entire time the shaft assembly is in between the first and second ends of the stroke. The first and second portions of the shaft assembly have a first material (such as Iron (Fe37), stainless steel, brass, bronze, aluminum or copper), and the third portion has a second material (metallic or non-metallic).

The sensor 110 senses the presence of the metallic material to determine when the shaft assembly reaches the ends of stroke. Specifically, the sensor 110 senses the metallic material in close proximity and makes a first indication, and the sensor 110 does not sense the metallic material in close proximity and makes a second indication, different from the first indication. When the sensor 110 makes the first indication, the controller 120 sends a signal to the valve 45, which moves the valve 45 between first and second positions. In the first and second positions one of the motive fluid chambers 30 a, 30 b is in communication with the source of motive fluid 42 while placing the other of the motive fluid chambers 30 a, 30 b in communication with the exhaust assembly 50. When the sensor 110 makes the second indication, the controller 120 maintains the valve 45 in the current position until the sensor 110 again makes the first indication.

In this regard, the motive fluid 42, the master valve 45, the motive fluid chambers 30 a, 30 b, and the diaphragms 20 a, 20 b work together as a drive assembly to drive reciprocating motion of the shaft 35. Other drive assemblies could be utilized in place of the illustrated drive assembly without departing from the scope of the present invention. Any other drive mechanism suitable for moving a shaft in reciprocating motion can be used.

Since the stroke distance is fixed, the volume of fluid pumped per stroke is fixed. Thus, for a given volumetric demand for fluid, a number of strokes can be calculated. As shown schematically in FIG. 7, the controller 120 actuates the master valve 45 (which may include a pilot valve and power valve or a single valve depending on the construction) to simultaneously place one of the motive fluid chambers 30 a, 30 b in communication with the source of motive fluid 42 while placing the other of the motive fluid chambers 30 a, 30 b in communication with the exhaust assembly 50. The illustrated master valve 45 is a four-way, three-position, center-position blocked valve. The master valve 45 is moveable to a first position (shown in FIG. 7) in which the motive fluid chamber 30 a is fluidly coupled with the source of motive fluid 42, and the motive fluid chamber 30 b is fluidly coupled with the exhaust assembly 50. The master valve 45 is moveable to a second position in which the motive fluid chamber 30 a is fluidly coupled with the exhaust assembly 50 and the motive fluid chamber 30 b is fluidly coupled with the source of motive fluid 42.

The controller 120 communicates (via wire or wirelessly) with the sensor 110 to actuate or toggle the master valve 45 only upon the shaft 35 reaching the ends of its stroke to ensure precise and consistent displacement of pumped fluid per stroke. When a volumetric demand for pumped fluid is received by the controller 120 from an operator or automatic system, the controller 120 initiates operation of the master valve 45 and operates the pump 10 for the number of strokes required to deliver the volume demanded. In an alternative embodiment, the controller 120 initiates operation of the master valve 45 and operates the pump 10 for the number of strokes required to deliver the volume demanded.

The illustrated master valve 45 is moveable to a center “off” position in which the motive fluid chambers 30 a, 30 b are substantially or completely closed off from either the motive fluid source 42 and the exhaust assembly 50 (i.e., the illustrated valve 45 is a three-position four-way valve). In another embodiment, the master valve 45 does not include a center “off” position (i.e., the valve 45 can take the form of a two-position four-way valve). In still other embodiments, the master valve 45 includes an “off” position at one end or the other of the valve, but not in the center. The master valve 45 can include any of a plurality of actuators, such as a spring, a solenoid, a push button, lever, cam roller, or any combination thereof.

In the second embodiment, illustrated in FIGS. 8-11, a sleeve 125 is inserted into a notch 130 in the shaft 35. The sleeve 125 includes a non-metallic or insulating material, such as a plastic, ceramic, or other suitable material. In some embodiments, the sleeve 125 is metallic, but does not include any of iron, stainless steel, brass, bronze, aluminum or copper. The outer diameter of the sleeve 125 is about equal to that of the shaft 35 in the illustrated embodiment. The shaft 35 is made of a metallic material and is proximate the sensor 110 when the shaft 35 is at the ends of its stroke, and thus senses the metallic material of the shaft 35 in close proximity to the sensor 110. When the shaft 35 is moving between the first and second ends of the stroke, however, the sleeve 125 is interposed between the sensor 110 and the shaft 35 such that the sensor 110 is not proximate the shaft 35, and such that the sensor 110 does not generate the proximity signal.

In this embodiment, the first and second portions of the shaft assembly include the portions of the shaft 35 adjacent the ends of the sleeve 125, and the third portion of the shaft assembly includes the sleeve 125 and the portion of the shaft 35 covered by the sleeve 125. When the shaft 35 is between the ends of the stroke, the sleeve 125 is positioned between the sensor 110 and the metallic material of the shaft 35, so that sensor 110 does not sense metallic material in close proximity in the middle of the stroke. Thus, in this embodiment, the sleeve 125 is proximate the sensor 110 the entire time that the shaft 35 is in between the ends of its stroke. The sensor 110 generates the proximity signal only when the shaft 35 is at the first and second ends of the stroke, and generates no proximity signal the entire time that the shaft 35 is in between the first and second ends.

Variations on the two illustrated embodiments are within the scope of the present invention. For example, instead of using an air gap at the ends of the stroke in the first embodiment, non-metallic or insulating sleeves (similar to the sleeve 125) may be positioned around the ends of the shaft 35. The second embodiment may be modified to provide notches 130 and sleeves on the portions of the shaft 35 that are proximate the sensor 110 when the shaft 35 is at the ends of its stroke, and the portion of the shaft 35 proximate the sensor 110 in between the ends of the stroke may be of standard diameter such that it is proximate the sensor 110 when the shaft 35 is in between the first and second ends of the stroke. Other variations of the two illustrated embodiments are also possible (e.g., constructing the shaft out of non-conductive material and using rings or sleeves of conductive material around the shaft), provided that the sensor 110 is either proximate or not proximate a metallic material when at the ends of the stroke and is the opposite (not proximate or proximate, respectively) a metallic material when the shaft 35 is in between the ends of its stroke.

The shaft 35 and the sleeve 125 together form a shaft assembly. The shaft assembly includes first and second portions that have a first material. The shaft assembly has a third portion that has a second material, different from the first material. The shaft 35 and the metallic ring 115 together form a shaft assembly. In the embodiment of FIGS. 8-11, the first and second portions include the first and second ends of the shaft 35, in which the first material is metallic, such as iron, stainless steel, brass, bronze, aluminum or copper. The illustrated third portion includes the region between the first and second ends in which the second material is any material that is different than the first material. The second material can be polymeric, ceramic, metallic, even including the metals listed above, as long as the second material is different than the first material.

As discussed above, the sensor 110 senses the difference in materials to determine when the shaft assembly reaches the ends of stroke. Specifically, the sensor 110 senses the proximity of a first material and makes a first indication, and the sensor 110 senses the proximity of a second material and makes a second indication, different from the first indication. When the sensor 110 makes the first indication, the controller 120 sends a signal to the valve 45, which simultaneously places one of the motive fluid chambers 30 a, 30 b in communication with the source of motive fluid 42 while placing the other of the motive fluid chambers 30 a, 30 b in communication with the exhaust assembly 50. When the sensor 110 makes the second indication, the controller 120 maintains the valve 45 in the current position until the sensor 110 makes the first indication.

In some embodiments, the first material can be magnetic and the second material can be non-magnetic. In other embodiments, the first material can have a first magnetic property whereas the second material has a second magnetic property, different from the first magnetic property. In some embodiments, the first material can be electrically conductive and the second material can be electrically non-conductive or insulating. In still other embodiments, the first material can have different conductive properties than the second material. Other similar variations between the first and second materials are possible and are considered to be within the scope of the present invention.

FIG. 12 illustrates an alternative valve configuration for the master valve, including a first valve 45 a and a second valve 45 b. The first and second valves 45 a, 45 b move simultaneously or at substantially the same time between first positions and second positions. The first position is illustrated in FIG. 12. When in the first position, the first valve 45 a fluidly couples the motive fluid chamber 30 b with the exhaust assembly 50 and the second valve 45 b fluidly couples the motive fluid chamber 30 a with the motive fluid source 42. When in the second position, the first valve 45 a fluidly couples the motive fluid chamber 30 a with the exhaust assembly 50 and the second valve 45 b fluidly couples the motive fluid chamber 30 b with the motive fluid source 42. Other various valve configurations for the master valve 45 or for first and second valves 45 a, 45 b can be utilized. For example, the first and second valves 45 a, 45 b can be interchanged or replaced with other three-way, two-position valves in other embodiments. The illustrated valves are given by way of example only and are not intended to limit the scope of the present invention.

Although in the embodiments described above, the sensor is either on (i.e., generates the signal) or off (i.e., does not generate the signal), in other embodiments the sensor may generate signals of different frequency, wavelength, magnitude and/or other characteristic to indicate whether or not the sensor is proximate a metallic material. In all embodiments, however, the sensor may be said to make a first indication when at the ends of the stroke and a second indication, different from the first indication, when in between the ends of the stroke. The first indication may include generating or not generating the proximity signal, the second indication may include the opposite of the first indication (i.e., respectively not generating or generating the proximity signal). In other embodiments, the first indication may include generating a first signal having a first signal characteristic and the second indication may include generating a second signal having a second signal characteristic different from the first signal characteristic. Also, in all embodiments of the invention, the same single sensor making both the first and second indications; the invention does not require multiple sensors to monitor whether the reciprocating shaft is at the ends or in between the ends of the stroke.

While embodiments and applications of this invention have been shown and described, it will be apparent to those skilled in the art that many more modifications are possible without departing from the inventive concepts herein described. It is understood, therefore, that the invention is capable of modification and therefore is not to be limited to the precise details set forth. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims without departing from the spirit of the invention.

Various features and advantages of the invention are set forth in the following claims. 

1. A prime mover comprising: a shaft assembly supported for reciprocal movement between first and second ends of a stroke, the shaft assembly include first and second portions having a first material and a third portion in between the first and second portions having a second material different from the first material; and a single inductive proximity sensor making a first indication in response to being proximate the first material and making a second indication different from the first indication in response to being proximate the second material; wherein a portion of the single inductive proximity sensor is proximate the first portion of the shaft assembly when the shaft is at the first end of the stroke; wherein the same portion of the same single inductive proximity sensor is proximate the second portion of the shaft assembly when the shaft is at the second end of the stroke; and wherein the same portion of the same single inductive proximity sensor is proximate the third portion of the shaft assembly the entire time the shaft is in between the first and second ends of the stroke.
 2. The prime mover of claim 1, further comprising: a source of motive fluid; an exhaust assembly; first and second motive fluid chambers, the shaft moving to the first end of the stroke in response to the second motive fluid chamber being in communication with the source of motive fluid and the first motive fluid chamber being in communication with the exhaust assembly, and the shaft moving to the second end of the stroke in response to the first motive fluid chamber being in communication with the source of motive fluid and the second motive fluid chamber being in communication with the exhaust assembly; a master valve in communication with the source of motive fluid and shiftable between a first condition in which the master valve places the source of motive fluid in communication with the second motive fluid chamber and places the first motive fluid chamber in communication with the exhaust assembly to cause shaft assembly movement to the first end of the stroke, and a second condition in which the master valve places the source of motive fluid in communication with the first motive fluid chamber and places the second motive fluid chamber in communication with the exhaust assembly to cause shaft assembly movement to the second end of the stroke; and a controller communicating with the single inductive proximity sensor to receive the first and second indications, and communicating with the master valve to switch the master valve between the first and second conditions each time the controller receives the first indication from the single inductive proximity sensor.
 3. The prime mover of claim 1, wherein the first indication includes generation of a proximity signal; and wherein the second indication includes generation of no signal.
 4. The prime mover of claim 1, wherein the second indication includes generation of a proximity signal; and wherein the first indication includes generation of no signal.
 5. The prime mover of claim 1, wherein the first and second portions of the shaft assembly include notches to create an air gap between the single inductive proximity sensor and the shaft assembly; wherein the third portion of the shaft assembly includes a metallic material proximate the single inductive proximity sensor; wherein the first indication includes generation of no signal; and wherein the second indication includes generation of a proximity signal.
 6. The prime mover of claim 1, wherein the shaft assembly includes a shaft having a first outer diameter and a ring having a second outer diameter larger than the first outer diameter; wherein the ring is affixed to and covers a portion the shaft; wherein the ring is constructed of metallic material; wherein the third portion of the shaft assembly is defined by the ring and the portion of the shaft covered by the ring; wherein the first and second portions of the shaft assembly include portions of the shaft adjacent to opposite ends of the ring and not covered by the ring; wherein the metallic material of the ring in the third portion is proximate the single inductive proximity sensor when the shaft assembly is in between the first and second ends of the stroke; and wherein an air gap is interposed between the shaft and the single inductive proximity sensor when the shaft assembly is at the first and second ends of the stroke such that the single inductive proximity sensor is not proximate the shaft.
 7. The prime mover of claim 1, wherein the shaft assembly includes a shaft constructed of metallic material and a sleeve constructed of non metallic material covering a portion of the shaft; wherein the third portion of the shaft assembly is defined by the sleeve and the portion of the shaft covered by the sleeve; wherein the first and second portions of the shaft assembly include portions of the shaft adjacent to opposite ends of the sleeve and not covered by the sleeve; wherein the metallic material of the shaft in the first and second portions is proximate the single inductive proximity sensor when the shaft assembly is at the respective first and second ends of the stroke; and wherein the sleeve is interposed between the shaft and the single inductive proximity sensor when the shaft assembly is in between the first and second ends of the stroke.
 8. A method of operating a machine, the method comprising: providing a shaft assembly comprising first and second portions having a first material and a third portion having second material different from the first material; providing a drive assembly operable to drive reciprocation of the shaft assembly between first and second opposite ends of a stroke; providing a single inductive proximity sensor; positioning the first portion of the shaft assembly proximate the single inductive proximity sensor when the shaft assembly is at the first end of the stroke; positioning the second portion of the shaft assembly proximate the single inductive proximity sensor when the shaft assembly is at the second end of the stroke; positioning the third portion of the shaft assembly proximate the single inductive proximity sensor the entire time the shaft assembly is in between the first and second ends of the stroke; making a first indication with the single inductive proximity sensor in response to the single inductive proximity sensor being proximate the first and second portions of the shaft assembly; making a second indication different from the first indication with the single inductive proximity sensor in response to the single inductive proximity sensor being proximate the third portion of the shaft assembly; and actuating the drive assembly to reverse a direction of movement of the shaft assembly in response to the single inductive proximity sensor making the first indication.
 9. The method of claim 8, wherein making the first indication includes generating a proximity signal; and wherein making the second indication includes generating no signal.
 10. The method of claim 8, wherein making the second indication includes generating a proximity signal; and wherein making the first indication includes generating no signal.
 11. The method of claim 8, wherein providing a shaft assembly includes creating an air gap between the single inductive proximity sensor and the shaft assembly in the first and second portions, and including a metallic material proximate the single inductive proximity sensor in the third portion; wherein making the first indication includes generating no signal; and wherein making the second indication includes generating a proximity signal.
 12. The method of claim 8, wherein providing a shaft assembly includes providing a shaft having a first outer diameter and a metallic ring having a second outer diameter larger than the first outer diameter; wherein providing a shaft assembly further includes affixing the ring to a portion the shaft and covering the portion of the shaft with the ring to define the third portion of the shaft assembly by the ring and the portion of the shaft covered by the ring; wherein providing a shaft assembly further includes defining the first and second portions of the shaft assembly with portions of the shaft adjacent opposite ends of the ring and not covered by the ring; wherein positioning the third portion of the shaft assembly proximate the single inductive proximity sensor includes positioning the metallic material of the ring proximate the single inductive proximity sensor when the shaft assembly is in between the first and second ends of the stroke; and wherein positioning the first and second portions of the shaft proximate the single inductive proximity sensor includes interposing an air gap between the shaft and the single inductive proximity sensor when the shaft assembly is at the first and second ends of the stroke such that the single inductive proximity sensor is not proximate the shaft.
 13. The method of claim 8, wherein providing a shaft assembly includes providing a shaft constructed of metallic material and a sleeve constructed of non metallic material covering a portion of the shaft; wherein providing a shaft assembly further includes defining the third portion of the shaft assembly by the sleeve and the portion of the shaft covered by the sleeve; wherein providing a shaft assembly further includes defining the first and second portions of the shaft assembly with portions of the shaft adjacent to opposite ends of the sleeve and not covered by the sleeve; wherein the steps of positioning the first and second portions of the shaft assembly proximate the single inductive proximity sensor include positioning the metallic material of the shaft in the first and second portions proximate the single inductive proximity sensor when the shaft assembly is at the respective first and second ends of the stroke; and wherein the step of positioning the third portion of the shaft assembly proximate the single inductive proximity sensor includes interposing the sleeve between the shaft and the single inductive proximity sensor the entire time the shaft assembly is in between the first and second ends of the stroke. 